The present invention relates to a liquid phase sintered body of a carbonitride alloy with titanium as main component which has improved properties particularly when used as cutting tool material in cutting operations requiring sharp edges in combination with high wear resistance and plastic deformation resistance. This has been achieved by heat treating the material in a nitrogen atmosphere.
Titanium-based carbonitride alloys, so-called cermets, are today well established as insert material in the metal cutting industry and are especially used for finishing. They comprise carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of other composition.
In addition to titanium, group VIa elements, normally both molybdenum and tungsten and sometimes chromium, are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, i.e., Zr, Hf, V, Nb and Ta, are also added in all commercial alloys available today. All these additional elements are usually added as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase is normally a solid solution of mainly both cobalt and nickel. The amount of binder phase is generally 3-25 wt %. Other elements are sometimes added as well, e.g., aluminum, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase.
One main advantage with cermets compared to WC-Co-based material is that relatively high wear resistance and chemical inertness can be obtained without applying surface coatings. This property is utilized mainly in extreme finishing operations requiring sharp edges and chemical inertness to cut at low feed and high speed. However, these desirable properties are generally obtained at the expense of toughness and edge security as well as ease of production. The most successful materials have a large nitrogen content (N/(C+N) often exceeding 50%) which makes sintering in conventional processes difficult due to porosity caused by denitrification. The high nitrogen content also makes the material difficult to grind. Grinding may be necessary to obtain sharp defect free edges and close tolerances. Ideally, for extreme finishing operations, one would like to have an uncoated cermet with low to moderate nitrogen content for ease of production, but with a wear resistance essentially the same as PVD- or CVD-coated material.
U.S. Pat. No. 4,447,263 discloses inserts of a titanium-based carbonitride alloy provided with a wear resistant surface layer of carbonitride or oxycarbonitride alone or in combination where the surface layer is completely free from binder phase. The layer is obtained by a heat treatment at 1100-1350.degree. C. in an atmosphere of N.sub.2, CO and/or CO.sub.2 at subpressure.
Another example is in U.S. Pat. 5,336,292 where the surface layer contains a low amount of binder phase but is separated from the interior of the material by a sharp interface to a binder phase enriched zone. The layer is obtained by heat treatment in an atmosphere of N.sub.2 and/or NH.sub.3 possibly in combination with at least one of CH.sub.4, CO and CO.sub.2 at 1100-1350.degree. C. for 1-25 hours at atmospheric pressure or higher.